This invention relates to efficient dielectric heaters for relatively non-conductive materials.
There are several benefits to an efficient process for recycling discarded plastics, such as polystyrene and styrofoam, into new, useful products. For example, consuming scrap plastic such as packing "peanuts" and styrofoam cups or dishes, rather than discarding the scrap plastic, has a beneficial impact on the environment by lessening the demand for landfill space. Additionally, recycling scrap may result in a lower demand for the raw materials and energy necessary to produce new plastic, once again benefiting the environment.
One known end use for recycled plastics is the creation of fire-retardant construction materials. For example, U.S. Pat. No. 4,596,682 discloses a plastic polymer for bonding shredded styrofoam chips into molded foam insulating panels and blocks for the construction industry. A significant feature of the polymer is that it produces a fire-retardant foam product, as opposed to high flammability typical of polystyrene and styrofoam.
Molded plastic items may be manufactured by creating a mold of the desired item introducing plastic resin into the mold, and heating the plastic resin until it begins to "cure" (heating the plastic until an exothermic reaction commences). One method of heating the plastic is to apply radiant heat. However, in practice, it was found that radiant heat sources simply cured or charred the outer surface of the plastic, but left the inside uncured. Thus, a different form of heating was required.
Another method of curing recycled plastics in a mold is to apply non-radiant heating. One form of non-radiant heating is microwave heating which heats primarily by agitating water molecules (which are resonant at 2.45 GHz) in the material to be heated. Microwave heating subjects poorly conductive materials to antenna-launched electromagnetic energy at microwave frequencies, typically at frequencies of about 2.45 GHz. U.S. Pat. No. 3,848,038 discloses an example of heating nonconductive materials with microwave energy. The thickness, size, and composition of material to be heated, however, limits the applicability of microwave heating.
Another form of non-radiant heating is dielectric heating. Similar to microwave heating, dielectric heating occurs when an electrically non-conductive material is subjected to radiofrequency ("RF") energy. However, dielectric electric field, that is, between flat parallel plates of a capacitor, rather than an antenna, and at lower frequencies, typically 3-30 MHz, than microwave heating.
In dielectric heating, the electric field is generated by applied equal and opposite potentials on the two opposing metal plates of a capacitor. In a typical mold, the capacitor plates form the sides of the mold. A radio frequency power source ("RF source") is applied to the capacitor plates. Typically, one plate and one side of the RF source are at ground, and the other plate is connected to the high side of the RF source. The material to be molded is inserted between two plates of a capacitor. When the molding compound is a plastic resin, the dielectric heating will raise the resin to its polymerization point, triggering an exothermic reaction and hardening the resin.
One problem with known dielectric heating is poor efficiency. For example, one known dielectric heater uses a 20 kilowatt RF source. This known dielectric heater puts a workpiece between the plates of a capacitor comprising a parallel resonant tank circuit of a vacuum tube power oscillator. However, the electrical impedance of the capacitor is low in comparison to the high impedance of a typical vacuum tube plate circuit. This impedance mismatch results in poor efficiency.
Another problem with known dielectric heating is a practical limit to the size of mold. Any given mold has a resonant frequency that depends on the area of the plates of the capacitor that form the mold. Efficient transfer of power from the RF source to the mold occurs when the RF source is tuned to oscillate at the resonant frequency of the mold. The larger the area of the mold, the lower the resonant frequency. Thus, large molds resulted in low resonant frequencies, and therefore required low frequency RF sources. However, plastics are typically more efficient at absorbing high frequency RF energy than low frequency RF energy. Thus, large size, low resonant frequency molds were inefficient in transmitting RF energy to the plastic to be heated. In practice, molds, and therefore finished pieces, were typically limited to approximately to 24 inches by 48 inches.
Additionally, the electrical properties of a mold comprise a complex impedance, with the capacitive reactance far exceeding the resistive component. A significant disadvantage caused by this impedance is that only the small resistive components cause the desired dielectric heating via E-field losses. Also, the large capacitive component typically resulted in a significant impedance mismatch between the mold and the RF source.
One aspect of the present invention is to provide a novel dielectric heater including a RF source having a first impedance, a parallel plate mold having a second impedance, and an impedance network, matching the first impedance to the second impedance, connected to the RF source and the parallel plate mold. The impedance network may be connected in series with the RF source and the parallel plate mold. The RF source may include a RF terminal and a common terminal, the impedance network may include a first terminal, connected to the RF terminal, and a second terminal, and the parallel plate mold may include a first plate, connected to the second terminal of the impedance network, and a second plate, connected to the common terminal of the RF source.
Another aspect of the present invention is to provide a novel dielectric heater including a RF source and a parallel plate mold, where the parallel plates are divided into a number of series-connected cells. According to this aspect, the series cells lower the capacitive component, and may be configured such that an impedance of the parallel plate mold approximates an impedance of the RF source.
Another aspect of the present invention is to provide a dielectric heater including a RF source and a parallel plate mold, where the parallel plates are divided into a number of series-connected cells, and an impedance network to match the impedance of the RF source to the series-connected cells.